1. Field of the Invention
The present invention relates to novel hexafluoro-1,3-butadiene prepolymer, curable materials using the same and laminates comprising the curable materials and more particularly, to materials for print circuit substrates having excellent heat resistance, moisture resistance and electric properties.
The present invention also relates to a fluorine-containing photo-curable resin composition, and varnish and prepreg using the resin composition as well as their use.
2. Statement of the Prior Art
For polymerization of hexafluoro-1,3-butadiene, radical polymerization and anion polymerization have been investigated. In the former radical polymerization, polymerizable composition occurs at random so that the polymerization product is a mixture of 1,2-polymer and 1,4-polymer (U.S. Pat. No. 3,663,521) (Journal of Polymer Science, Part B, 6, 639-642, 1968). The latter anion polymerization has been considered as an effective means for obtaining polymer having excellent steric regularity; however, it has been pointed out that when an alkyl lithium, Grignard reagent, an alkoxide or the like is used as a reaction initiator, the yield of polymer is as extremely poor as less than 1%.
For this reason, it is the actual situation that in spite of realizing a low water absorption and a low moisture vapor permeability thereby to expect improvement in moisture resistance, decrease in a dielectric constant, improvement in the surface quality, imparting heat resistance, etc., hexafluoro-1,3-butadiene polymer has not been applied to curable materials which are obtained by impregnating fibrous substrates including a glass prepreg, a glass mat, etc. with the polymer.
On the other hand, severe requirements for laminate materials used in multilayer circuit substrates for electron devices have been made to reduce their dielectric constant, from viewpoints of improving reliability on moistureproof and reliability on heat resistance and increasing computation speed of computers. As a means therefor, it has been investigated to apply fluorine materials (Japanese Patent Application Laid-Open No. 62-29454). The main trend of investigations on fluorine materials is toward thermoplastic saturated fluorocarbon materials but it has not been reported to apply thermosetting fluorine materials.
Thermoplastic saturated fluorine materials encounter many problems that material fluidity in laminate processing is poor, a thermal expansion coefficient is large due to a low glass transition point of molded products, dimensional stability in molded laminate products which is the key in achieving a minute circuit is inferior, wettability to fibrous substrates such as glass, Kevlar, etc. is poor to worsen impregnation property, it is difficult to improve mechanical strength, etc. It is thus difficult to apply thermoplastic fluorine materials to laminate materials.
As insulating materials having a specific dielectric constant of less than 3, there are hitherto known fluorine resins represented by polytetrafluoroethylene, tetrafluoroethylene-hexafluoropropylene copolymer, etc.; hydrocarbon resins such as polyethylene, polystyrene, polypropylene, etc. These insulating materials are generally used widely.
Fluorine resins are flame retardant as compared to hydrocarbon resins; however, as stated above fluorine resins are thermoplastic and inferior in mechanical property and dimensional stability at high temperature. To solve the problems, fluorine resins such as polytetrafluoroethylene as described above are incorporated into thermosetting resins which can form a crosslinking structure.
For example, there is known a method which comprises incorporating fluorine resin such as polytetrafluoroethylene, etc. into a resin composition comprising epoxy-modified polybutadiene, epoxy resin, carboxylic acid anhydride and a radical polymerization initiator, thereby to reduce a dielectric constant of insulating resin (Japanese Patent Application Laid-Open No. 2-72700).
Further in a process for preparing a laminate which comprises laminating resin boards obtained by impregnating a reinforcing material of the board with a fluorocarbon resin and adhering the boards, a 3-dimensionally crosslinked fluorocarbon resin layer is formed on the surface of the boards by plasma polymerization or sputtering, etc. so that an insulating material is improved in its mechanical property and dimensional stability at high temperature and the dielectric constant is reduced (Japanese Patent Application Laid-Open No. 2-106344).
However, even though saturated fluorine resin such as polytetrafluoroethylene, etc. is incorporated into conventional insulating resin such as epoxy resin or polybutadiene resin to attempt to reduce its dielectric constant, the insulating material has merely a poor affinity to fluorine resin. Furthermore, saturated fluorine resin such as polytetrafluoroethylene, etc. has a high melting point and is insoluble in a solvent; it is thus difficult to prepare varnish. Accordingly, an uneven portion is caused in the resin layer after curing. In addition, due to curing by heating, there are problems in dimensional accuracy and dimensional stability. For these reasons, mismatching tends to occur which makes it difficult to use these mixed resins as an insulating material for multilayered printing plate particularly requiring high density and high accuracy.
Further in the prior art methods by means of plasma polymerization or sputtering, resins are synthesized in the gaseous phase so that an efficiency of forming a resin layer is poor. To reach a desired thickness, a long period of time is required. In addition, crosslinking occurs at random since it is difficult to control the crosslinking reaction, and a thermal expansion coefficient is large due to free volume in the resin. Furthermore, high vacuum is required for plasma polymerization. Therefore, plasma polymerization or squttering results in poor productivity.